1. Field of the Invention
The present invention relates to an OFDM receiver which receives an OFDM signal transmitted from an OFDM transmitter by using a sub-carrier.
2. Description of the Related Art
Referring to FIG. 1, description is made of a configuration of a conventional orthogonal frequency division multiplexing (OFDM) transmitter (OFDM transmitter of a conventional technology 1, hereinafter) 100.
As shown in FIG. 1, the conventional OFDM transmitter 100 of the conventional technology 1 mainly includes an encoder section 101, an interleaver section 102, a mapping section 103, an IFFT section 104, and a guard interval addition section 105.
The encoder section 101 is configured to execute error correction encoding processing for an input information signal (information bit).
The interleaver section 102 is configured to execute interleave processing for the information signal output from the encoder section 101, and to output the signal to the mapping section 103.
The mapping section 103 is configured to map the information signal output from the interleaver section 102 in a symbol.
For example, when 16 QAM is used as a modulation system, the mapping section 103 maps four “0, 1” signals in one symbol constituted of 16 points on an IQ plane.
The mapping section 103 is configured to map the symbol in a plurality of sub-carriers, and to output the sub-carriers to the IFFT section 104. Here, the plurality of sub-carriers are orthogonal to each other in frequency.
The IFFT section 104 is configured to execute IFFT (inverse fast Fourier transformation) processing for the symbols mapped in the plurality of sub-carriers which have been output from the symbol mapping section 103 on a predetermined FFT window, and to output a transmission signal of a time domain.
For example, as shown in FIG. 1, the transmission signal of the time domain contains signal components “s(1) to s(4)”. Here, “s(k)” indicates a transmitted signal component (symbol), and “k” indicates an index which shows a symbol transmission order before guard interval addition. Incidentally, as shown in FIG. 1, each signal component “s(k)” is formed by signal components corresponding to the plurality of sub-carriers.
The guard interval addition section 105 is configured to copy a part of the transmission signal output from the IFFT section 104 (e.g., signal components “s(3) and s(4)”), and to add the copy to the transmission signal of the time domain.
Here, a part of the copied transmission signal of the time domain (e.g., signal components “s(3) and s(4)”) is equivalent to a “guard interval”.
The transmission signal of the time domain (OFDM signal, hereinafter) to which the guard interval has been added is transmitted from an antenna of the OFDM transmitter 100 to an OFDM receiver.
Next, referring to FIG. 2, description will be made of a configuration of the OFDM receiver 200 of the conventional technology 1. As shown in FIG. 2, the conventional OFDM receiver 200 mainly includes an FFT section 201, a linear filter section 202, a filter generation section 203, a demapping section 204, a deinterleaver section 205, and a decoder section 206.
The FFT section 201 is configured to remove a guard interval from the OFDM signal transmitted from the OFDM transmitter 100. Subsequently, as described later, the FFT section 201 is configured to execute FFT (Fast Fourier Transformation) processing for the OFDM signal on a set FFT window, and to output a signal of a frequency domain corresponding to each sub-carrier.
FIG. 3 shows signal components of the OFDM signal received by the FFT section 201. FIG. 3 shows an example in which the FFT section 201 receives the OFDM signal from the OFDM transmitter 100 through three multi-paths #0 to #2. In the example of FIG. 3, the number of FFT points (FFT window size) is “4”, and the number of guard interval points (guard interval length) is “2”.
In this case, “s(k)” indicates a transmitted signal component, and “k” indicates an index which indicates a transmission symbol transmission order before guard interval addition. “h(1)” indicates a signal component received through the first multi-path #1.
The real OFDM signal in the FFT section 201 becomes a sum of all the signal components at each point of time, i.e., a total of the signal components of all rows of FIG. 3 (for each column).
Ideally, in order to detect signal components which constitute a specific symbol, the FFT section 201 must set an FFT window so as not to contain signal components which constitute previous and subsequent symbols.
By setting the FFT window in the above manner, in each of the OFDM signals received through the multi-paths #0 to #2, each row in the FFT window contains signal components (e.g., “s(1) to s(4)”) constituting a target symbol. Accordingly, orthogonality of the sub-carriers can be maintained.
Even when the signal components constituting the target symbol (e.g., “s(1) to s(4)”) are cyclically shifted in the FFT window, the orthogonality of the sub-carriers can be maintained.
As a result, a channel of each sub-carrier can be regarded as flat fading in the OFDM receiver 200.
A signal of a frequency domain corresponding to each sub-carrier is subjected to compensation processing for channel variation through the linear filter section 202 and the filter generation section 203.
The demapping section 204 is configured to execute demapping processing for the signal of the frequency domain corresponding to each sub-carrier which has been output from the linear filter section 202, and to output the information signal to the deinterleaver section 205.
The deinterleaver section 205 is configured to execute deinteleaving processing for the information signal output from the demapping section 204, and to output the information signal to the decoder section 206.
The decoder section 206 is configured to execute error correction decoding processing for the information signal output from the deinterleaver section 205, thereby reproducing the information signal input to the OFDM transmitter 100.
However, in the OFDM receiver 200 of the conventional art 1, when an impulse response length exceeds a guard interval length, a problem occurs. Referring to FIG. 4, the problem will be described.
In an example of FIG. 4, i.e., in an example in which the FFT section 201 receives the OFDM signal through the four multi-paths #0 to #4, unlike the case of FIG. 3, the FFT section 201 cannot set an FFT window so as not to contain signal components of previous and subsequent symbols when signal components of a specific symbol are detected.
Consequently, for the signal components of the specific symbol, inter-symbol interference (ISI) occurs due to the signal components of the previous and subsequent symbols.
Additionally, in the example of FIG. 4, the OFDM signal received through the multi-path #3 is not formed in a manner that only signal components (e.g., “s(1) to s(4)”) only of a target symbol are contained in the FFT window.
Consequently, orthogonality of the sub-carriers is disturbed (because the signal components “s(1) to s(4)” are not even in a form of being cyclically shifted), and inter-carrier interference (ICI) occurs by the adjacent sub-carriers.
Generally, as it can prevent deterioration of reception characteristics through the multi-paths, an OFDM transmission system is an effective transmission system especially in a wide band transmission in which a multi-path influence becomes conspicuous.
However, in the OFDM transmission system, a guard interval length to be added must be set longer than a channel impulse response length. In the case of “(guard interval length+1 FFT point length)<(channel impulse response length)”, in addition to the occurrence of inter-symbol interference caused by the multi-path influence, the orthogonality of the sub-carriers is lost. Thus, there is a problem in that inter-carrier interference also occurs.
In this regard, in the OFDM transmission system, when a guard interval length is set long by imagining a longest impulse response length whose probability is low but which may occur depending on surrounding situations, frequency use efficiency is reduced.
Thus, a transmission system has been requested which can prevent deterioration of reception characteristics caused by inter-symbol interference and inter-carrier interference, even when a channel impulse response length exceeds the guard interval length.
To solve the problem, i.e., as countermeasures when the channel impulse response length exceeds the guard interval length, an OFDM receiver of a conventional technology 2 has been presented.
FIG. 5 shows a configuration of an OFDM receiver 200 of the conventional technology 2. The entire configuration of the OFDM receiver 200 of the conventional technology 2 is a turbo-equalization receiver. Incidentally, in place of the FFT processing, MMSE filtering processing is used to convert a signal of a time domain into a signal of a frequency domain.
Specifically, as shown in FIG. 5, the OFDM receiver 200 of the conventional technology 2 includes an ISI compensation section 300, an ICI compensation section 400, a channel estimation section 208, a linear filter section 202, a filer generation section 203, a demapping section 204, a deinterleaver section 205, a decoder section 206, and a transmission signal estimated value obtaining section 207.
The channel estimation section 208 is configured to obtain channel estimated values of the multi-paths #1 to #3 based on the OFDM signals received through the plurality of multi-paths #0 to #3 (FIG. 8), and to transmit the obtained channel estimated values (including impulse response length) to the ISI compensation section 300 and the ICI compensation section 400.
The ISI compensation section 300 is configured to execute ISI compensation processing for the signal received from the OFDM transmitter 100, based on the channel estimated value from the channel estimation section 208 and a transmission signal estimated value from the transmission signal estimated value obtaining section 207.
Specifically, as shown in FIG. 6, the ISI compensation section 300 includes an ISI component selection section 302, a channel simulator section 303, and a subtraction section 304.
The ISI component selection section 302 is configured to select a signal component likely to cause inter-symbol interference from among the transmission signal estimated value from the transmission signal estimated value obtaining section 207 based on the impulse response length from the channel estimation section 208, and to output the signal component to the channel simulator section 303.
In an example of FIG. 8, based on the impulse response length from the channel estimation section 208, the ISI component selection section 302 selects a signal component “s(4−Ns)” received after a delay exceeding a guard interval from among the transmission signal estimated value from the transmission signal estimated value obtaining section 207, as a signal component likely to cause inter-symbol interference, and outputs the signal component to the channel simulator section 303.
The channel simulator section 303 is configured to convolute a channel impulse response in the signal component from the ISI component selection section 302 based on the channel estimated value from the channel estimation section 208, so as to obtain a replica indicating an interference signal component to be canceled, and to output the replica to the subtraction section 304.
In the example of FIG. 8, the channel simulator section 303 convolutes a channel impulse response of the multi-path #3 in the signal component “s(4−Ns)” from the ISI component selection section 302, so as to obtain a replica “h(3)s(4−Ns)”, and to output the replica to the subtraction section 304.
The subtraction section 304 is configured to subtract the replica (“h(3)s(4−Ns)” in the example of FIG. 8) output from the channel simulator section 303 from the received OFDM signal, so as to obtain an OFDM signal after ISI compensation, and to output the OFDM signal to the ICI compensation section 400.
The ICI compensation section 400 is configured to execute ICI compensation processing for the OFDM signal after the ISI compensation from the ISI compensation section 300, based on the channel estimated value from the channel estimation section 208 and the transmission signal estimated value from the transmission signal estimated value obtaining section 207.
Specifically, as shown in FIG. 7, the ICI compensation section 400 includes an ICI compensation section 4001 for a sub-carrier #1 to an ICI compensation section 400n for a sub-carrier #n. The ICI compensation sections 4001 to 400n are all similar in structure, and thus the ICI compensation section 4001 only for the sub-carrier #1 is described.
As shown in FIG. 7, the ICI compensation section 4001 for the sub-carrier #1 includes an undesired transmission signal estimated value selection section 401, an ICI component selection section 403, a channel simulator section 404, a subtraction section 405, a guard interval removal section 406, and a sub-carrier component extraction section 407.
The undesired transmission signal estimated value selection section 401 is configured to convert, in a frequency domain, transmission signal estimated values (signals of a time domain) from the transmission signal estimated value obtaining section 207 into signals of the frequency domain, to select transmission signal estimated values (signals of the frequency domain) corresponding to the sub-carriers #2 to #n other than the sub-carrier #1 from the transmission signal estimated values, to convert the selected transmission signal estimated values (signals of the frequency domain) into signals of the time domain, and to output the signals to the ICI component selection section 403.
Based on an impulse response length from the channel estimation section 208, the ICI component selection section 403 is configured to select signal components likely to cause inter-carrier interference from among the transmission signal estimated values corresponding to the sub-carriers #2 to #n from the undesired transmission signal estimated value selection section 401, and to output the signal components to the channel simulator section 404.
In the example of FIG. 8, based on the impulse response length from the channel estimation section 208, the ICI component selection section 403 extracts a multi-path #3 which does not become a form (including a cyclically shifted form) containing signal components “s(1) to s(4)” only which constitute a target symbol in the FFT window used by the OFDM transmitter 100.
Subsequently, the ICI component selection section 403 selects the signal components “s(3), s(4) and s(1)” in the FFT window of the OFDM signal received through the multi-path #3, as signal components likely to cause inter-carrier interference, and outputs the signal components “s(3), s(4) and s(1)” to the channel simulator section 404.
The channel simulator section 404 is configured to convolute a channel impulse response in the signal components from the ICI component selection section 403 based on the channel estimated value from the channel estimation section 208, so as to obtain replicas indicating interference signal components to be canceled, and to output the replicas to the subtraction section 405.
In the example of FIG. 8, the channel simulator section 404 obtains replicas “h(3)s(3), h(3)s(4) and h(3)s(1)” by convoluting the channel impulse response of the multi-path #3 in the signal components “s(3), s(4) and s(1)” of those from the ICI component selection section 403, and outputs the replicas “h(3)s(3), h(3)s(4) and h(3)s(1)” to the subtraction section 304.
The subtraction section 405 is configured to obtain a signal by subtracting the replicas (in the example of FIG. 8, “h(3)s(3), h(3)s(4) and h(3)s(1)” from the channel simulator section 404, from the OFDM signal after the ISI compensation, and to output the obtained signal to the guard interval removal section 406.
The guard interval removal section 406 is configured to remove a guard interval from the signal sent from the subtraction section 405, and to output the signal to the sub-carrier component extraction section 407.
FIG. 9 shows signal components contained in signals output from the guard interval removal section 406.
As shown in FIG. 9, the signals output from the guard interval removal section 406 contain signal components “h(0)s(1) to h(0)s(4)”, “h(1)s(1) to h(1)s(4)”, and “h(2)s(2) to h(2)s(4)” corresponding to all the sub-carriers regarding the signals received through the multi-paths #0 to #2, and signal components “h(3)s(3), h(3)s(4) and h(3)s(1)” corresponding to the sub-carrier #1 regarding the signals received through the multi-path #3.
The sub-carrier component extraction section 407 is configured to multiply the signal output from the guard interval removal section 406 by a row vector constituted of 1st line elements of a DFT (Discrete Fourier Transformation) matrix described below, so as to calculate a signal of a frequency domain (OFDM signal after ICI compensation) corresponding to the sub-carrier 1, and to output the signal of a frequency domain to the linear filter section 202.
      F    =          [                                    1                                1                                                                                          …                                                                                          1                                                1                                w                                              w              2                                                                                                                                                                              w              N                                                                                                                                    w              2                                                                                                      ⋰                                                                                                                                                                    ⋮                                ⋮                                                                                                                                                                  w                                                (                                      i                    -                    1                                    )                                ×                                  (                                      j                    -                    1                                    )                                                                          ⋮                                                                                                                                                                                                                                                                                        ⋰                                                                                                          1                                              w              N                                                                                                      …                                                                                                                                                  ]        ,      w    =          ⅇ                        -          j                ⁢                              2            ⁢            π                    N                    
According to the conventional technology 2, the sub-carrier component extraction section 407 includes an MMSE filter.
Incidentally, in order to reproduce information signals by taking signal components of OFDM signals received through all the multi-paths #0 to #3 into consideration, the ICI sub-carrier compensation section 400 is configured not to cancel all the signal components “(h (3) s(x)” of the OFDM signal received through the multi-path #3 from among the OFDM signals after ISI compensation, but to cancel signal components “h(3)s(x)” only corresponding to the sub-carrier other than a specific sub-carrier from among the signal components “h (3) s(x)” of the OFDM signal received through the multi-path #3.
The transmission signal estimated value obtaining section 207 is configured to execute processing similar to the error correction encoding processing, interleaving processing, symbol mapping processing, guard interval adding processing, and IFFT processing of the OFDM transmitter 100 for the information signal reproduced by the decoder section 206, so as to calculate a transmission signal estimated value which is an estimated value of the OFDM signal sent from the OFDM transmitter 100, and to output the transmission signal estimated value to the ISI compensation section 300, the ICI compensation section 400, the linear filter section 202 and the filter generation section 203.
However, in the conventional OFDM receiver 200, a replica must be generated for each sub-carrier since the information signals are reproduced by taking the signal components of the OFDM signals received through all the multi-paths into consideration. Consequently, there is a problem in that the amount of processing for ICI compensation becomes very large.
Furthermore, in the conventional OFDM receiver 200, since the information signals are reproduced by taking the signal components of the OFDM signals received through all the multi-paths into consideration, ICI compensation processing is executed for each sub-carrier. Consequently, FFT (Fast Discrete Fourier Transformation) processing cannot be used, so as to create a problem in that the amount of processing becomes very large.